Significance
Infants discriminate speech sounds universally until 8 mo of age, then native discrimination improves and nonnative discrimination declines. Using magnetoencephalography, we investigate the contribution of auditory and motor brain systems to this developmental transition. We show that 7-mo-old infants activate auditory and motor brain areas similarly for native and nonnative sounds; by 11–12 mo, greater activation in auditory brain areas occurs for native sounds, whereas greater activation in motor brain areas occurs for nonnative sounds, matching the adult pattern. We posit that hearing speech invokes an Analysis by Synthesis process: auditory analysis of speech is coupled with synthesis that predicts the motor plans necessary to produce it. Both brain systems contribute to the developmental transition in infant speech perception.